This invention relates to RF switching devices, and particularly to switching devices which use non-reciprocal components, such as three and four port circulators in combination with switchable three and four port circulators, hereafter referred to as three and four port switches, respectively. The purpose of combining such building blocks is to effect the connection of an input terminal to one of several output terminals in a reciprocal manner.
FIGS. 1 and 2 illustrate two types of known ferrite switches in schematic form. FIG. 1 illustrates a three-port switch, which includes a ferrite element interacting with the RF (microwave magnetic) field and results in non-reciprocal switching of an input signal at port P1, to either of two output ports, labelled P2 and P3, respectively. Three port switches of the type shown in FIG. 1 may be constructed using waveguide junctions with the ferrite element located near the center of the junction, or they may be constructed using any transmission line junction, such as coaxial lines, stripline, or microstrip. The ferrite element is magnetically biased by a permanent magnet in the circulator version, whereas in the switch version the internal magnetic flux direction is reversed by an external electromagnet, or a closed loop magnetic geometry may be left in either remanent magnetic state of its major or minor hysteresis loop. These two states of internal magnetic flux orientation will result in two switch states when the ferrite is placed in the microwave junction.
In a junction circulator the interaction between the magnetized ferrite and the microwave (R.F.) magnetic field may be viewed as a differential permeability interacting with two counter-rotating microwave magnetic fields, into which a linearly polarized R.F. magnetic field may be resolved. In a four port switch, the R.F. magnetic field in rectangular waveguide may be considered as locally circularly polarized, thus removing the degeneracy, a requirement of non-reciprocal interaction in the microwave circulator or switch. FIG. 2 schematically illustrates the four port device function based on the theory outlined above. The ports of the four port circulator or switch include ports on opposite sides of the schematic diagram of the four port device, which are decoupled, or isolated, from each other. Thus, in the four port device shown in FIG. 2, ports P1 and P3 are decoupled, and ports P2 and P4 are decoupled, from each other. Remaining pairs of ports are non-reciprocally coupled to each other.
The four port circulator or switch can be implemented by a pair of hybrid junctions, such as a magic tee and sidewall hybrid pair in waveguide, or ring hybrid and junction hybrids in TEM configurations. Between such pairs of hybrid junctions, sections of transmission line are loaded with ferrite configurations and/or diectric configurations so that non-reciprocal interaction between the R.F. magnetic field and the internal magnetic flux of the ferrite may occur. This arrangment results in differential phase shift which, in turn, in combination with the hybrid junction, results in non-reciprocal port connection described schematically above. Those skilled in the art will recognize that four port configurations can be obtained by combining two three port devices. When a four port switch is used to connect one transmitter and one receiver to two antennas, alternately, such operation may be undesirable in certain system applications. For antennas which are stationary or only movable over a limited angular range, two transmission lines may be used to their respective antennas. When such antennas are used in applications requiring greater angular coverage, a two-channel rotary joint is required. This may present a difficult design problem in high power systems. When the switch is placed past the rotary joint, the dual channel rotary joint needs to have only one high power and one low power channel. The insertion loss of the low power channel may be unacceptable from a system sensitivity viewpoint.
The above difficulties are compounded when more than two antennas are used. With a reciprocal ferrite switch, transmission and reception paths are identical so that the switch can be mounted between the multiple antennas and a single channel rotary joint. This arrangement is especially attractive in high frequency, high power systems, where multiple channel rotary joints are not feasible.
It has been known in the art to provide reciprocal switching, which avoids the need to switch between transmitter and receiver operation by providing a ferrite phase shifter operated switching arrangement, wherein the ferrite is switched between a neutral, reciprocal state and a second state which provides all of the required phase differential (usually 180.degree.) in a single channel. This results in a design with undesired high insertion loss, giving particular problems in the high power applications.
It is an object of the present invention to provide a reciprocal RF switch for selectively and reciprocally connecting an input terminal to a selected one of a plurality of output terminals using non-reciprocal components.